The present invention concerns that part of a combine harvester or threshing machine mechanism usually known as the cleaning shoe and whose function is to receive crop material from the threshing and separating sections of the machine and screen or sieve the material so as to isolatge clean grain for delivery to a grain tank or other receptacle.
The "conventional" cleaning shoe configuration is well known. Its basic elements are an upper screen (the chaffer), a lower screen (the sieve), a blower for delivering an air blast longitudinally and upwardly to the underside of the screens, means for delivering a flow of threshed and partially separated crop material to a forward part of the chaffer and means for oscillating or reciprocating the screens so that material on them migrates rearwardly. Clean grain passing downwards through both screens is collected and taken away by a clean grain conveyor. "Tailings", including unthreshed heads and some heavier, larger pieces of trash, intercepted by the sieve, fall from the rearward edge of the sieve and are taken away by a tailings conveyor for rethreshing. Lighter trash, including chaff, is discharged to the ground from the rear of the cleaning shoe by the air blast from the blower.
A major objective in combine design is to balance the capacity and efficiency of the principal components--those responsible for gathering, threshing, separating and cleaning. The wide range of crop type and operating conditions make this difficult. Moreover, cost considerations, transporting limitations and a need for good maneuverability make higher volumetric efficiencies for the vehicle as a whole very desirable. For a given nominal combine capacity, the overall dimensions of the machine must be minimized.
Combine capacity is typically expressed in terms of material flow for a given level of grain loss. The cleaning shoe is often a critical or limiting factor with its performance essentially establishing the effective capacity of the combine. Within the shoe itself, a balance must be maintained between the respective capacities of the chaffer and sieve, given that practical considerations lead them to be generally of similar dimensions and disposed one above the other. Within the shoe, the function of the chaffer (the upper screen) is perhaps most sensitive. In the conventional arrangement, the chaffer must deal with a largely undifferentiated heterogeneous mixture of grain, straw and chaff and other debris and, within its length, achieve a sorting of material so that a maximum of grain passes through to the sieve below and a minimum of grain is carried over the end of the chaffer. Unfortunately, among the wide range of types and conditions of crop material that a combine must handle, are many which tend to "overload the chaffer" by dumping onto it such a mat of material that throughput (rate of work) must be seriously reduced in order to keep grain losses within acceptable limits.
Many commercial combines still use the conventional shoe with little or no variation but the patent and other literature record many departures from the conventional, aimed at increasing volumetric efficiency. The essence of a number of these variations is to increase the effective chaffer surface area in relation to sieve area. The additional chaffer area is usually provided with an upwardly and rearwardly directed air supply, possibly ducted independently of the air supply to the main portion of the chaffer. However, typically, the potential for increased cleaner volumetric efficiency represented by more chaffer area is not realized because all material passing downwards from the chaffer portions is still delivered to a sieve which it has not been feasible to expand and which may thus become the "bottle neck" in the cleaning shoe.
In the variations referred to above attempts are made to provide additional screen area or make other modifications within the space of a given conventional combine configuration. Among the more promising of these is the conversion of the grain pan (usually extending forwardly of the shoe and above the cleaning shoe blower) from a simple conveying function to that of combined processing and conveying. A good example of this is disclosed by Ferguson in U.S. Pat. No. 2,228,228 but Ferguson also still delivers all material passing down from his extended chaffer to a single sieve screen of essentially conventional dimensions.